In the prior art methods, blow molded articles are produced by extruding a hollow tube of a thermoplastic material in a molten state (hereinafter referred to as "parison"), and clamping said parison within a mold and blowing a blowing gas into the parison to expand the parison through a blow pin inserted into the mold. It is known to employ a reactive gas such as fluorine or a blowing cas containing a reactive component such as fluorine to improve the fluid impermeability of the resulting blow molded thermoplastic articles such as blow molded containers; see U.S. Pat. No. 3,862,284, the description of which is incorporated herein by reference. The method disclosed in this patent comprises blowing a parison with a treating gas containing 0.1 to 10% by volume fluorine and 99.9 to 90% by volume inert gas to form an article of, for example, polyethylene, in the mold and then after a cooling stage, purging the gas within the article with an inert gas.